1. Field of the Invention
This invention relates to a device and a method for measuring the thermal conductivity of thin films of materials, and more particularly a method and device for measuring the thermal conductivity of rigid or flexible, homogeneous or heterogeneous, thin films between 50 .mu.m and 150 .mu.m thick with relative standard deviations of less than five percent.
2. Description of the Related Art
NASA is developing Temperature Sensitive Paints (TSP's) for global non-intrusive detection of boundary layer transition in flow over the surface of wind tunnel research models. The TSP sensitivity should be large enough to resolve the smallest amplitude and the highest frequency fluctuations, since the transition process involves unsteady fluctuations. Based on linear steady-state heat transfer analysis, one of the steps to improve the paint sensitivity is minimizing the thermal conductivity of the paint. The TSP applied to wind tunnel research models is typically 25 .mu.m to 125 .mu.m thick with an additional 25 .mu.m thick primer layer. The TSP's are typically composed of metal complexes in polymer binders. A thermal conductivity measuring device is needed to measure thermal conductivity of the TSP's and accompanying primer layer, which are typically between 50 .mu.m and 150 .mu.m total thickness.
Existing thermal conductivity measuring devices are suitable only for very thin (&lt;20 .mu.m) or very thick (&gt;6.25 mm) specimens. Ultrasonic principles have been used to measure film thicknesses less than 20 .mu.m, but such principles require a priori knowledge of the material's specific heat and density to determine the thermal conductivity. These devices target silicon dioxide films suitable for microelectronics, micromechanics, micro-optics, and semiconductor processing. Devices marketed to measure thicker specimens (&gt;6.25 mm), such as insulations, composites, cloth, natural fibers such as wood, generally have difficulty attaining one-dimensional conduction. The alternating current technique measures thermal conductivity of bulk gases, but a modified technique measures thin films and is referred to as the 3-.omega. technique, discussed in D. G. Cahill, H. E. Fischer, T. Klitsner, E. T. Swartz, and R. O. Pohl, "Thermal Conductivity of Thin Films: Measurements and Understanding", American Vacuum Society, 1989. Other techniques have also been used to make thin film thermal conductivity measurements: thermal comparators, such as described in R. W. Powell, "Experiments using a simple thermal comparator for measurement of thermal conductivity, surface roughness and thickness of foils or surface deposits", J. Sci. Instrum., 1957; specialized film geometries, such as described in B. T. Boiko, A. T. Pugachev, and V. M. Bratsychin, "Method for the determination of the thermophysical properties of evaporated thin films", Thin Solid Films, 1973; laser calorimetry, such as set forth in D. Ristau, and J. Ebert, "Development of a thermographic laser calorimeter", Appl. Opt., 1986; and flash radiometry, such as set forth in N. Tsutsumi, and T. Kiyotsukuri, "Measurement of thermal diffusivity for polymer film by flash radiometry", Appl. Phys. Lett., 1988. The steady-state test methods described in ASTM Standards E1530-93 and D5470-95 are applicable to stacked thin-film specimens that are homogeneous. Stacking would be required for thin films 50 .mu.m 150 .mu.m thick. Thermal conductivity varies with thickness so stacking introduces measurement inaccuracies. The steady state test method described in ASTM Standard E1225 is applicable to homogeneous, opaque specimens and has a thermocouple design, which introduces inaccuracies into the thermal conductivity measurement.
The conventional types of thermal conductivity meters have the problems of heat losses, contact resistance losses, and large inaccuracies. Furthermore, existing mathematical (empirical) models for determining the thermal conductivity of thin films are not very reliable, especially over a wide range of pressures and temperatures. Therefore, a measurement technique is needed to measure, both steady state and transient, the thermal conductivity of thin films of materials, such as paints, that are 50-150 .mu.m thick, with relative standard deviations of less than five percent.